In several species of Xiphophorus (Poeciliidae, Pisces) a gene locus, P, controls the age at which the gonadotropes differentiate and become physiologically active. Since gonad maturation depends upon gonadotropins, this gene also determines the age of sexual maturity. Thirdly, this gene causes significant differences in size of adult males, since their growth rate declines sharply under the influence of androgens. We are planning to determine the number of alleles per species and different kinds of allelic interaction. In X. pygmaeus this gene locus also controls growth rates in both sexes. The pituitary gland will be examined to determine whether this is mediated via the somatotropes. BIBLIOGRAPHIC REFERENCES: Klaus D. Kallman, 1975. The Platyfish, Xiphophorus maculatus, In Handbook of Genetics, vol. 4, R.C. King, editor, pp. 81-132. Plenum Publishing Corp. H.J. Sobel, E. Marquet, K.D. Kallman and G.J. Corley, 1975. Melanomas in Platy/Swordtail Hybrids, In the Pathology of Fishes, W.E. Ribelin and G. Migaki, editors, pp. 945-981. The University of Wisconsin Press.